Golf ball

ABSTRACT

In a golf ball comprising a core, a cover layer, an intermediate layer between the core and the cover layer, the cover layer is formed of a resin material composed primarily of polyurethane or polyurea, a sphere obtained by encasing the core with the intermediate layer (intermediate layer-encased sphere) has a lightness (Li value) in the L*a*b* color space of at least 25 on its surface, and the cover layer has a visible light transmittance of at least 0.6%. The golf ball has a surface with a distinctive aesthetic appearance and retains both a good spin performance and a good scuff resistance, making it useful as a ball for professional and skilled amateur golfers.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of copending application Ser. No. 16/951,028 filed on Nov. 18, 2020, claiming priority based on Japanese Patent Application No. 2019-226473 filed in Japan on Dec. 16, 2019, the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a golf ball with a polyurethane cover and an intermediate layer-encased sphere having a lightness (Li value) of at least 25 on its surface. More specifically, the invention relates to a golf ball that has a distinctive aesthetic appearance and is able to retain both a good spin performance and a good scuff resistance.

BACKGROUND ART

Golf balls are required to possess, among other characteristics, a good flight and stopping performance and a good scuff resistance. Golf balls have thus been developed so as to fly well on shots with a driver and to be suitably receptive to backspin on approach shots. Cover materials with a high resilience and a good scuff resistance have been developed to date with this in mind. Recently, golf balls in which the cover material is a polyurethane material that is superior in backspin performance on approach shots are favored by professional golfers and skilled amateurs. Also, aside from ordinary white golf balls, colored balls too have become popular among golfers.

Polyurethane resins, owing to their molecular structure, are generally translucent or cloudy in the uncolored state. To impart color, a pigment or dye is mixed in and kneaded with the resin. However, depending on the hue to which the resin is being colored, heating occurs during kneading of the resin composition and chemical reactions among constituents within the polyurethane resin itself arise, resulting in poor color development and a dull coloration. Also, when a resin material that has been colored with a dye is used, during golf ball manufacturing operations, the dye migrates into the resin-transporting lines and the like that are used, leading to undesirable outcomes such as contamination of the production equipment.

In terms of colored golf balls in which the cover material is made of polyurethane, yellow-colored and orange-colored balls already exist on the market. Although these colored golf balls do address a demand for colors other than white, they are ultimately colored balls for skilled golfers that are aimed at attributes such as visibility and spin performance.

Methods for coloring the cover material of a golf ball that have already been disclosed include methods in which a pigment or dye is mixed into a cover-forming resin material, and methods in which the surface of the cover is color treated. See, for example, the following literature: JP-A 2016-513488, JP-A 2016-513489, U.S. Patent Application No. 2014/256468, U.S. Patent Application No. 2014/250609, U.S. Patent Application No. 2018/080172, JP-A 2004-180921, JP-A S58-143769 and JP-A S58-112567.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a golf ball wherein the cover material includes a polyurethane resin as the base resin, which golf ball is endowed with the excellent scuff resistance and spin performance required by professional golfers and skilled amateurs, and also has a distinctive aesthetic appearance and highly visible.

As a result of extensive investigations, we have found that, in a golf ball comprising a core, a cover layer, an intermediate layer between the core and the cover layer, the cover layer being formed of a polyurethane resin material, by adjusting the lightness (Li value) of the surface of a sphere obtained by encasing the core with the intermediate layer (intermediate layer-encased sphere) to at least 25 on its surface and adjusting the visible light transmittance of the cover layer to at least 0.6%, the surface of the ball has a distinctive aesthetic appearance, plus the ball is able to maintain a good spin performance and good scuff resistance, making it useful as a golf ball for professional golfers and skilled amateurs.

Accordingly, the invention provides a golf ball comprising a core, a cover layer, an intermediate layer between the core and the cover layer, wherein the cover layer is formed of a resin material composed primarily of polyurethane or polyurea, a sphere obtained by encasing the core with the intermediate layer (intermediate layer-encased sphere) has a lightness (Li value) in the L*a*b* color space of at least 25 on its surface, and the cover layer has a visible light transmittance of at least 0.6%.

In a preferred embodiment, the ball has a lightness (Lo value) in the L*a*b* color space of at least 25. In this case, it is preferable that the ratio of the lightness (Lo value) and the lightness (Li value) of the intermediate layer-encased sphere, expressed by Lo/Li is from 0.3 to 1.5.

In another preferred embodiment, the resin material of the cover layer has a Shore D hardness of not more than 65.

In a further preferred embodiment, the cover layer is formed of a resin material composed primarily of polyurethane in which a polyol component includes poly(tetramethylene glycol) having the number-average molecular weight of not more than 3,500.

Advantageous Effects of the Invention

In the golf ball of the invention, the surface of the ball has a distinctive aesthetic appearance and the ball is able to retain both a good spin performance and a good scuff resistance, making it useful as a ball for professional golfers and skilled amateurs.

BRIEF DESCRIPTION OF THE DIAGRAMS

FIG. 1 is a schematic cross-sectional view of a golf ball according to one embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The objects, features and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the appended diagram.

The golf ball of the invention is a ball having a core, a cover layer, an intermediate layer between the core and the cover layer. The cover layer is corresponding to an outermost layer. The outermost layer (the cover layer) is formed of a resin material composed primarily of polyurethane or polyurea.

Referring to FIG. 1, the golf ball G of the invention has, for example, a core 1, an intermediate layer 2 and a cover layer (outermost layer) 3. The surface of the cover layer 3 has numerous dimples D formed thereon. As to FIG. 1, the reference symbol 4 represents a coating layer that is applied to the surface of the cover layer, and the reference symbol 2a is a color agent included in the intermediate layer 2, which is described below.

The core is generally obtained by vulcanizing a rubber composition composed primarily of a base rubber. The core may be formed using, as this rubber composition, one which includes, for example, a base rubber, a co-crosslinking agent, a crosslinking initiator, a metal oxide and an antioxidant. Polybutadiene is preferably used as the base rubber of to this rubber composition.

Next, the intermediate layer is described.

In the present invention, a sphere obtained by encasing the core with the intermediate layer (intermediate layer-encased sphere) has a lightness (Li value) that is at least 25, preferably at least 30, more preferably at least 35, further more preferably 35, and most preferably at least 45. The lightness (Li value) means L* value (the lightness) based on the L*a*b* color space of HS Z8781. When this value is high, the intermediate layer-encased sphere has a bright color; when it is low, the intermediate layer-encased sphere has a dark color.

In order to make an intermediate layer-encased sphere have a predetermined lightness, well-known resin materials and various additives such as pigments are adopted as a resin composition for the intermediate layer by adjusting a suitable weight ratio of the above ingredients, but there is no particular limitation. Preferably, at least one intermediate layer is formed of a resin composition that includes (A) and (B) below:

(A) a thermoplastic resin, and

(B) a colorant consisting of a fluorescent dye or a fluorescent pigment.

Components (A) and (B) are described below.

(A) Thermoplastic Resin

Exemplary thermoplastic resins include, without particular limitation, resins that have hitherto been used as golf ball materials, such as ionomeric resins, polyester resins, polyurethane resins, polyamide resins, polyolefin resins, olefin-based thermoplastic elastomers and styrene-based thermoplastic elastomers. Ionomeric resins are especially suitable and preferably include either of (a) and (b) below:

-   (a) an ethylene-α,β-unsaturated carboxylic acid copolymer and/or a     metal salt thereof, -   (b) an ethylene-α,β-unsaturated carboxylic acid-α,β-unsaturated     carboxylic acid ester copolymer and/or a metal salt thereof.

Specific examples of the α,β-unsaturated carboxylic acid in components (a) and (b) include acrylic acid, methacrylic acid, maleic acid and fumaric acid. Acrylic acid and methacrylic acid are especially preferred. The α,β-unsaturated carboxylic acid ester in component (b) is preferably a lower alkyl ester of the above unsaturated carboxylic acid, specific examples of which include methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, methyl acrylate, ethyl acrylate, propyl acrylate and butyl acrylate. Butyl acrylate (butyl n-acrylate, butyl i-acrylate) is especially preferred.

Metal ion neutralization products of the copolymers in components (a) and (b) can be obtained by partially neutralizing acid groups on the olefin-unsaturated carboxylic acid copolymer or the olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester copolymer with metal ions. Illustrative examples of metal ions which neutralize the acid groups include Na⁺, K⁺, Li⁺, Zn⁺⁺, Cu⁺⁺, Mg⁺⁺, Ca⁺⁺, Ni⁺⁺ and Pb⁺⁺ Preferred use can be made of Na⁺, Li⁺, Zn⁺⁺, Mg⁺⁺ and Ca⁺⁺ in particular. Such neutralization products may be obtained by a known method. For example, a neutralization product may be obtained by using, for reaction with the above copolymer, a compound such as a formate, acetate, nitrate, carbonate, bicarbonate, oxide, hydroxide or alkoxide of the above metal ion.

Known substances may be used as components (a) and (b). Illustrative examples include commercial products such as the following acid copolymers: Nucrel® N1560, Nucrel® N1214, Nucrel® N1035, Nucrel® AN4221C, Nucrel® AN4311, Nucrel® AN4318 and Nucrel® AN4319 (all products of DuPont-Mitsui Polychemicals Co., Ltd.). Illustrative examples of metal ion neutralization products of acid copolymers include Himilan® 1554, Himilan® 1557, Himilan® 1601, Himilan® 1605, Himilan® 1706, Himilan® AM7311, Himilan® 1855, Himilan® 1856 and Himilan® AM7316 (all products of DuPont-Mitsui Polychemicals Co., Ltd.), and Surlyn® 7930, Surlyn® 6320, Surlyn® 8320, Surlyn® 9320 and Surlyn® 8120 (E.I. DuPont de Nemours and Company).

The overall amount of thermoplastic resin serving as component (A) is not particularly limited, although it is recommended that the thermoplastic resin be included in an amount which is typically at least 70 wt %, preferably at least 80 wt %, and more preferably at least 90 wt %, of the total amount of the resin composition. When enough is not included, the desired effects of the invention may not be achievable.

(B) Colorant Comprising Fluorescent Dye or Fluorescent Pigment

This invention is directed at a golf ball having a distinctive aesthetic appearance whose surface is transparent and soft-toned. A colorant consisting of a fluorescent dye or a fluorescent pigment is included for this purpose as component (B) in the intermediate layer-forming resin material. Color is imparted to the intermediate layer by suitably including a known fluorescent dye or fluorescent pigment as the colorant. Examples include solvent yellow, solvent orange, anthraquinone and phthalocyanine (all of which are dyes), and also yellow fluorescent pigments, pink fluorescent pigments and orange fluorescent pigments. Known commercial products may be used as these colorants.

In this invention, of fluorescent colorants, the use of one that is light harvesting is preferred. Light-harvesting fluorescent colorants are materials which have the ability to collect sunlight and convert the wavelength to the long-wavelength side as fluorescent light. These materials are characterized in that they collect light by totally reflecting it at the interior of the colored material and guiding it to the dimple edges, where the light is emitted in a concentrated state, and they produce intense coloration.

Such light-harvesting fluorescent colorants include systems that generate orange, pink, red, yellow, blue or violet colors. Commercial products may be used in any of these chromogenic systems. Examples of light-harvesting fluorescent dyes that may be used include those available from BASF under the trade names Lumogen F Yellow 083, Lumogen F Orange 240, Lumogen F Red 305 and Lumogen F Blue 650, and those available from Kashinomoto Technologies Co., Ltd. under the trade names Lumicolor Red, Smart Color LP Green, Smart Color LP Yellow and Smart Color LP Orange.

The amount of component (B) included per 100 parts by weight of component (A) is from 0.001 to 0.2 part by weight, and preferably from 0.005 to 0.1 part by weight. When this amount is low, the fluorescence may weaken and the desired decorativeness may not be obtained. On the other hand, when this amount is high, migration of the colorants, especially dyes, may arise, staining objects that come into contact with the golf ball.

(C) Inorganic Filler or Organic Filler

The resin composition of components (A) and (B) may further include, as component (C), an inorganic filler or an organic filler. The purpose of including this inorganic or organic filler is, as subsequently described, to suitably prepare the resin composition so that it has the desired transmittance in the visible light spectrum. However, component (C) is not an essential ingredient in this invention.

In cases where component (C) is an inorganic filler, illustrative examples include, without particular limitation, zinc oxide, barium sulfate, calcium carbonate, titanium dioxide and silica. Adding an inorganic filler makes it possible to impart translucency and to adjust to the color.

In cases where component (C) is an organic filler, illustrative examples include, without particular limitation, fine particles of crosslinked polymethyl methacrylate (crosslinked PMMA), crosslinked polybutyl methacrylate, crosslinked polyacrylate ester, crosslinked acrylic-styrene copolymer, melamine resin or polyurethane.

The amount of component (C) added per 100 parts by weight of component (A) is from 0.01 to 4.0 parts by weight, preferably from 0.02 to 3.0 parts by weight, and more preferably from 0.03 to 2.0 parts by weight. When too much is added, the hiding properties may become excessive, detracting from the decorativeness of a stylish ball, or the change in color when color fading occurs due to sunlight exposure may increase.

The resin composition can be obtained by mixing together the above ingredients using any of various types of mixers, such as a kneading-type single-screw or twin-screw extruder, a Banbury mixer or a kneader.

Various additives may be optionally included in the resin composition. For example, pigments, dispersants, antioxidants, light stabilizers, ultraviolet absorbers and lubricants may be suitably added.

The intermediate layer has a thickness which, although not particularly limited, is preferably at least 0.5 mm, and more preferably at least 0.7 mm, but preferably not more than 1.7 mm, and more preferably not more than 1.4 mm.

The intermediate layer has a hardness on the Shore D scale which, although not particularly limited, is preferably at least 30, and more preferably at least 40, but preferably not more than 75, more preferably not more than 70, and even more preferably not more than 65.

In this invention, the cover layer is formed of a resin material composed primarily of polyurethane or polyuria.

The polyurethane or polyurea serving as the resin material of the cover layer is described in detail below.

Polyurethane

The polyurethane has a structure which includes soft segments composed of a polymeric polyol (polymeric glycol) that is a long-chain polyol, and hard segments composed of a chain extender and a polyisocyanate. Here, the polymeric polyol serving as a starting material may be any that has hitherto been used in the art relating to polyurethane materials, and is not particularly limited. This is exemplified by polyester polyols, polyether polyols, polycarbonate polyols, polyester polycarbonate polyols, polyolefin polyols, conjugated diene polymer-based polyols, castor oil-based polyols, silicone-based polyols and vinyl polymer-based polyols. Specific examples of polyester polyols that may be used include adipate-type polyols such as polyethylene adipate glycol, polypropylene adipate glycol, polybutadiene adipate glycol and polyhexamethylene adipate glycol; and lactone-type polyols such as polycaprolactone polyol. Examples of polyether polyols include poly(ethylene glycol), poly(propylene glycol), poly(tetramethylene glycol) and poly(methyl tetramethylene glycol).

In this invention, it is preferable that the long-chain polyol includes poly tetramethylene glycol (PTMG) having the number-average molecular weight (Mn) of not more than 3,500 in view of workability of molding the golf balls, scuff resistance and rebound. The number-average molecular weight (Mn) of poly tetramethylene glycol (PTMG) is not more than 3,000, more preferably not more than 2,500, and most preferably not more than 2,000, but, the lower limit is preferably at least 800, more preferably at least 900, and most preferably at least 1,000.

Here and below, “number-average molecular weight” refers to the number-average molecular weight calculated based on the hydroxyl value measured in accordance with JIS-K1557.

The chain extender is not particularly limited; any chain extender that has hitherto been employed in the art relating to polyurethanes may be suitably used. In this invention, low-molecular-weight compounds with a molecular weight of 2,000 or less which have on the molecule two or more active hydrogen atoms capable of reacting with isocyanate groups may be used. Of these, preferred use can be made of aliphatic diols having from 2 to 12 carbon atoms. Specific examples include 1,4-butylene glycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol and 2,2-dimethyl-1,3-propanediol. Of these, the use of 1,4-butylene glycol is especially preferred.

Any polyisocyanate hitherto employed in the art relating to polyurethanes may be suitably used without particular limitation as the polyisocyanate. For example, use can be made of one or more selected from the group consisting of 4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, p-phenylene diisocyanate, xylylene diisocyanate, 1,5-naphthylene diisocyanate, tetramethylxylene diisocyanate, hydrogenated xylylene diisocyanate, dicyclohexylmethane diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, norbornene diisocyanate, trimethylhexamethylene diisocyanate, 1,4-bis(isocyanatomethyl)cyclohexane and dimer acid diisocyanate.

The ratio of active hydrogen atoms to isocyanate groups in the polyurethane-forming reaction may be suitably adjusted within a preferred range. Specifically, in preparing a polyurethane by reacting the above long-chain polyol, polyisocyanate and chain extender, it is preferable to use the respective components in proportions such that the amount of isocyanate groups included in the polyisocyanate per mole of active hydrogen atoms on the long-chain polyol and the chain extender is from 0.95 to 1.05 moles.

The method of preparing the polyurethane is not particularly limited. Preparation using the long-chain polyol, chain extender and polyisocyanate may be carried out by either a prepolymer process or a one-shot process via a known urethane-forming reaction. Of these, melt polymerization in the substantial absence of solvent is preferred. Production by continuous melt polymerization using a multiple screw extruder is especially preferred.

It is preferable to use a thermoplastic polyurethane material as the polyurethane. The thermoplastic polyurethane material may be a commercial product, examples of which include those available under the trade name Pandex from DIC Covestro Polymer, Ltd., and those available under the trade name Resamine from Dainichiseika Color & Chemicals Mfg. CO., Ltd.

Polyurea

The polyurea is a resin composition composed primarily of urea linkages formed by reacting (i) an isocyanate with (ii) an amine-terminated compound. This resin composition is described in detail below.

(i) Isocyanate

Suitable use can be made here of an isocyanate that is employed in the prior art relating to polyurethanes, although the isocyanate is not particularly limited. Use may be made of isocyanates similar to those described above in connection with the polyurethane material.

(ii) Amine-Terminated Compound

An amine-terminated compound is a compound having an amino group at the end of the molecular chain. In this invention, the long-chain polyamines and/or amine curing agents shown below may be used.

A long-chain polyamine is an amine compound which has on the molecule at least two amino groups capable of reacting with isocyanate groups, and which has a number-average molecular weight of from 1,000 to 5,000. In this invention, the number-average molecular weight is more preferably from 1,500 to 4,000, and even more preferably from 1,900 to 3,000. Examples of such long-chain polyamines include, but are not limited to, amine-terminated hydrocarbons, amine-terminated polyethers, amine-terminated polyesters, amine-terminated polycarbonates, amine-terminated polycaprolactones, and mixtures thereof. These long-chain polyamines may be used singly, or two or more may be used in combination.

An amine curing agent is an amine compound which has on the molecule at least two amino groups capable of reacting with isocyanate groups, and which has a number-average molecular weight of less than 1,000. In this invention, the number-average molecular weight is more preferably less than 800, and even more preferably less than 600. Specific examples of such amine curing agents include, but are not limited to, ethylenediamine, hexamethylenediamine, 1-methyl-2,6-cyclohexyldiamine, tetrahydroxypropylene ethylenediamine, 2,2,4- and 2,4,4-trimethyl-1,6-hexanediamine, 4,4′-bis(sec-butylamino)dicyclohexylmethane, 1,4-bis(sec-butylamino)cyclohexane, 1,2-bis(sec-butylamino)cyclohexane, derivatives of 4,4′-bis(sec-butylamino)dicyclohexylmethane, 4,4′-dicyclohexylmethanediamine, 1,4-cyclohexane bis(methylamine), 1,3-cyclohexane bis(methylamine), diethylene glycol di(aminopropyl) ether, 2-methylpentamethylenediamine, diaminocyclohexane, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, propylenediamine, 1,3-diaminopropane, dimethylaminopropylamine, diethylaminopropylamine, dipropylenetriamine, imidobis(propylamine), monoethanolamine, diethanolamine, triethanolamine, monoisopropanolamine, diisopropanolamine, isophoronediamine, 4,4′-methylenebis(2-chloroaniline), 3,5-dimethylthio-2,4-toluenediamine, 3,5-dimethylthio-2,6-toluenediamine, 3,5-diethylthio-2,4-toluenediamine, to 3,5-diethylthio-2,6-toluenediamine, 4,4′-bis(sec-butylamino)diphenylmethane and derivatives thereof, 1,4-bis(sec-butylamino)benzene, 1,2-bis(sec-butylamino)benzene, N,N′-dialkylaminodiphenylmethane, N,N,N′,N′-tetrakis(2-hydroxypropyl)ethylenediamine, trimethylene glycol di-p-aminobenzoate, polytetramethylene oxide di-p-aminobenzoate, 4,4′-methylenebis(3-chloro-2,6-diethyleneaniline), 4,4′-methylenebis(2,6-diethylaniline), m-phenylenediamine, p-phenylenediamine and mixtures thereof. These amine curing agents may be used singly or two or more may be used in combination.

(iii) Polyol

Although not an essential ingredient, in addition to the above-described components (i) and (ii), a polyol may also be included in the polyurea. The polyol is not particularly limited, but is preferably one that has hitherto been used in the art relating to polyurethanes. Specific examples include the long-chain polyols and/or polyol curing agents mentioned below.

The long-chain polyol may be any that has hitherto been used in the art relating to polyurethanes. Examples include, but are not limited to, polyester polyols, polyether polyols, polycarbonate polyols, polyester polycarbonate polyols, polyolefin-based polyols, conjugated diene polymer-based polyols, castor oil-based polyols, silicone-based polyols and vinyl polymer-based polyols. These long-chain polyols may be used singly or two or more may be used in combination.

The long-chain polyol has a number-average molecular weight of preferably from 1,000 to 5,000, and more preferably from 1,700 to 3,500. In this average molecular weight range, an even better rebound and productivity are obtained.

The polyol curing agent is preferably one that has hitherto been used in the art relating to polyurethanes, but is not subject to any particular limitation. In this invention, use may be made of a low-molecular-weight compound having on the molecule at least two active hydrogen atoms capable of reacting with isocyanate groups, and having a molecular weight of less than 1,000. Of these, the use of aliphatic diols having from 2 to 12 carbon atoms is preferred. Specific examples include 1,4-butylene glycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol and 2,2-dimethyl-1,3-propanediol. The use of 1,4-butylene glycol is especially preferred. The polyol curing agent has a number-average molecular weight of preferably less than 800, and more preferably less than 600.

A known method may be used to produce the polyurea. A prepolymer process, a one-shot process or some other known method may be suitably selected for this purpose.

The content of the above polyurethane or polyurea is suitably selected according to the required properties of the desired manufactured article, but is preferably at least 50 wt %, more preferably at least 60 wt %, and even more preferably at least 80 wt %, per 100 wt % of the overall resin composition that forms the cover layer. Resin materials other than polyurethane or polyurea may also be included in the resin composition that forms the cover layer, provided that doing so does not detract from the visibility and distinctive aesthetic appearance, the purpose of these being to, for example, further enhance the flowability of the golf ball resin composition, or to increase various properties of the golf ball such as the rebound and scuff resistance.

In addition, optional additives may be suitably included in the cover-forming resin composition. For example, inorganic fillers, organic staple fibers, reinforcing agents, crosslinking agents, pigments, dispersants, antioxidants, ultraviolet absorbers and light stabilizers may be added to the above ingredients. When such additives are included, the amount thereof per 100 parts by weight of the base resin is preferably at least 0.1 part by weight, and more preferably at least 0.5 part by weight. The upper limit is preferably not more than 10 parts by weight, and more preferably not more than 4 parts by weight.

The method of molding the cover layer is not particularly limited. Such molding may be carried out by injection molding, compression molding, casting, reaction injection molding (RIM) or some other known molding method. For example, the cover layer may be molded by feeding the above cover layer-forming resin composition into an injection molding machine and injecting the molten resin composition over the sphere obtained by encasing the core with an intermediate layer. In this case, the molding temperature differs depending on the type of polyurethane or polyurea serving as the base resin, but is typically in the range of 150 to 270° C.

The thickness of the cover layer is suitably selected according to the construction of the target golf ball, such as a three-piece golf ball. The thickness of this cover layer has an upper limit of generally not more than 3.0 mm, preferably not more than 2.0 mm, and more preferably not more than 1.5 mm. As for the lower limit in the thickness, a relatively thin cover layer results in a better spin performance on shots with a driver, but the scuff resistance worsens as the cover layer becomes thinner. Hence, to strike a good balance between both of these properties, the cover layer thickness has a lower limit of preferably at least 0.4 mm, more preferably at least 0.5 mm, and even more preferably at least 0.6 mm.

The material hardness of the cover layer-forming resin composition is suitably selected according to the spin properties and scuff resistance achievable by the golf ball. The material hardness, expressed on the Shore D hardness scale, is preferably not more than 65, more preferably not more than 60, and most preferably not more than 55. This is because both a high resin material transparency and a high spin performance can be obtained at a relatively low hardness. From the standpoint of moldability, the lower limit in the Shore D hardness is preferably at least 25, and more preferably at least 30.

Numerous dimples are generally provided on the surface of the layer serving as the outermost layer of the golf ball for reasons having to do with the aerodynamic performance of the ball. The number of dimples formed on the cover surface is not particularly limited. However, to enhance the aerodynamic performance and increase the distance traveled by the ball, the number of dimples is typically from 250 to 400.

In order for the golf ball to exhibit the distinctive aesthetic appearance that is desired in this invention, it is critical for the visible light transmittance of the cover layer to be at least 0.6%. This value is preferably at least 0.8%, more preferably at least 1.0%, and even more preferably at least 1.5%. To prevent that melting marks on the surface of the intermediate layer or the cover which arise during injection molding of the cover layer is seen through such that the desirable aesthetic appearance as the ball surface color is not obtained, the upper limit value in the visible light transmittance is preferably not more than 80%, more preferably not more than 50%, and most preferably not more than 20%. In this invention, the visible light transmittance of the cover layer can be measured using a commercial spectrophotometer in accordance with JIS K 0115. The visible light transmittance of the cover layer proper can be measured by peeling the cover layer that is the object of measurement from the golf ball, cutting out a thin strip in the shape in which measurement samples are mounted in the measuring instrument to be used and mounting the strip in the instrument.

The golf ball having a cover layer in this invention has a lightness (Lo value) in the L*a*b* color space of JIS Z8781 that is preferably at least 25, more preferably at least 30, even more preferably at least 35, and more preferably at least 40. The lightness (Lo value) means L* value (the lightness) based on the L*a*b* color space of JIS Z8781. When this value is high, the cover layer has a bright color; when it is low, the cover layer has a dark color.

In order to enhance the inventive desired effects described above, it is also preferable that the ratio of the lightness (Lo value) of the ball and the lightness (Li value) of the intermediate layer-encased sphere, as expressed by Lo/Li is preferably at least 0.3, more preferably at least 0.5, further more preferably at least 0.7, and most preferably at least 0.9. On the other hand, the upper limit of the above ratio is preferably at most 1.5, more preferably at most 1.3, further more preferably at most 1.2, and most preferably at most 1.1.

In the practice of this invention, a coat may be applied to the surface of the cover layer. The coating used when applying a coat to the surface of the cover layer is preferably a two-part curable urethane coating. The two-part curable urethane coating in this case includes a base resin composed primarily of a polyol resin and a curing agent composed primarily of a polyisocyanate. Such a coat is also called a clear coat.

When the coating is a two-part curable urethane coating, it is preferable to use as the base resin any of various polyols, such as saturated polyester polyols, acrylic polyols and polycarbonate polyols. It is also preferable to use as the isocyanate curing agent a non-yellowing polyisocyanate, examples of which include adducts, biurets, isocyanurates and mixtures thereof of hexamethylene diisocyanate, isophorone diisocyanate, hydrogenated xylylene diisocyanate and the like.

The coating contains the above resin as the base, to which various solvents and additives are suitably added, and may also include an effect pigment. When an effect pigment is used, it is preferable for the content of the effect pigment per 100 parts by weight of the base resin to be in the range of 1.0 to 10.0 parts by weight. When this content is too high, the ability of the spray gun to propel the coating material during spray painting decreases, which may make the coating operation more difficult to carry out. When the content is too low, the desired ball appearance may not be obtained.

Effect pigments are broadly divided into metal oxide-coated mica, basic lead carbonate, bismuth oxychloride and natural pearl essence. Of these, the selection of a metal oxide-coated mica is preferred because such pigments are nontoxic and have the best chemical stability. In general, titanium dioxide and iron oxide are commonly used as the metal oxide. By varying the coverage (thickness of the metal oxide coat), various colors and interference effects can be achieved. The larger the particle size of such pigments, the greater the degree of luster that can be achieved. However, at larger pigment particle sizes, the luster tends to subside. It is therefore necessary to select a pigment having a suitable particle size.

A golf ball that has been coated with a coating containing such an effect pigment is able to reflect light in many directions, increasing the quality feel of the ball. Moreover, because it fully reflects sunlight, the golf ball is easier to find.

The golf ball can also be fashioned into a ball having a so-called matte feel without luster by including delustering particles in the coating composition. The delustering particles used in this case are exemplified by silica particles, melamine particles and acrylic particles. Specific examples include particles made of silica, polymethyl methacrylate, polybutyl methacrylate, polystyrene or polybutyl acrylate. The particles may be either organic or inorganic particles, with the use of silica particles being especially preferred.

The content of delustering particles may be set to preferably from 5 to 10 parts by weight per 100 parts by weight of the base resin (combined amount of resin component and solvent) of the coating composition. When the content is too high, the viscosity of the coating composition rises, which tends to make the coating more difficult to apply. When the content is too low, the light-quenching effect may decrease.

A known method may be used without particular limitation as the method for applying the above coating onto the surface of the cover layer. Use can be made of a desired method, such as air-gun painting or electrostatic painting.

The thickness of the coating layer applied onto the cover layer, although not particularly limited, is generally from 8 to 22 μm, and preferably from 10 to 20 μm.

The weight, diameter and other ball specifications of the inventive golf ball may be suitably set in accordance with the Rules of Golf.

EXAMPLES

The following Examples and Comparative Examples are provided to illustrate the invention, and are not intended to limit the scope thereof.

Examples 1 to 17, Comparative Examples 1 to 6

A core-forming rubber composition formulated as shown in Table 1 and common to all of the Examples was prepared and then molded and vulcanized to produce a 38.6 mm diameter core.

TABLE 1 Rubber composition parts by weight cis-1,4-Polybutadiene 100 Zinc acrylate 27 Zinc oxide 4.0 Barium sulfate 16.5 Antioxidant 0.2 Organic peroxide (1) 0.6 Organic peroxide (2) 1.2 Zinc salt of pentachlorothiophenol 0.3 Zinc stearate 1.0

Details on the ingredients in the core material are given below.

-   cis-1,4-Polybutadiene: Available under the trade name “BR 01” from     JSR Corporation -   Zinc acrylate: Available from Nippon Shokubai Co., Ltd. -   Zinc oxide: Available from Sakai Chemical Co., Ltd. -   Barium sulfate: Available from Sakai Chemical Co., Ltd. -   Antioxidant: Available under the trade name “Nocrac NS6” from Ouchi     Shinko Chemical Industry Co., Ltd. -   Organic peroxide (1): Dicumyl peroxide, available under the trade     name “Percumyl D” from NOF Corporation -   Organic peroxide (2): A mixture of     1,1-di(tert-butylperoxy)cyclohexane and silica, available under the     trade name “Perhexa C-40” from NOF Corporation -   Zinc stearate: Available from NOF Corporation

Next, an intermediate layer-forming resin material in each of Examples and Comparative Examples was prepared. The intermediate layer-forming resin materials are shown in the following Table 2. This resin materials of the respective Examples were injection-molded over the 38.6 mm diameter core obtained as described above, thereby producing the intermediate layer-encased spheres having a 1.25 mm thick intermediate layer.

TABLE 2 Intermediate layer material (pbw) White Red Pink Yellow Green Himilan 1605 50 50 50 50 50 Himilan 1557 15 15 15 15 15 Himilan 1706 35 35 35 35 35 Titanium dioxide 3.78 1.89 0.030 0.030 Pigment (1) 0.01 0.01 Pigment (2) 0.08 0.04 Dye (1) 0.072 0.070 Dye (2) 0.010 Dye (3) 0.065 0.03 0.001 Magnesium stearate 0.9 0.9 0.9 0.9 0.9 Lubricant 0.05 0.05 0.05 0.05 0.05

Details on the materials in the above table are given below.

-   Himilan 1605: An ionomeric resin available from Dow-Mitsui     Polychemicals Co., Ltd. -   Himilan 1557: An ionomeric resin available from Dow-Mitsui     Polychemicals Co., Ltd. -   Himilan 1706: An ionomeric resin available from Dow-Mitsui     Polychemicals Co., Ltd. -   Pigment (1): Pigment Yellow 53 -   Pigment (2): Pigment Blue 29 -   Dye (1): Solvent Green 5 -   Dye (2): Available under the trade name “Smart Color LP Green 2”,     fluorescent dye, from Kashinomoto Technologies Co., Ltd. -   Dye (3) Available under the trade name “Lumicolor Red”, fluorescent     dye, from Kashinomoto Technologies Co., Ltd.

Lubricant: Available under the trade name “Sanwax 161-P” from Sanyo Chemical Industries, Ltd.

Next, the cover layer-forming resin materials for the respective Examples shown in Tables 4 to 6 below were then injection-molded over the intermediate layer-encased sphere, thereby producing uncoated golf balls (diameter, 42.7 mm) having a 0.8 mm thick cover layer (outermost layer). Dimples common to all the Examples were formed at this time on the cover surface of the balls in the respective Examples and Comparative Examples.

The golf balls in the respective Examples and Comparative Examples were then coated to a thickness of 15 μm on the cover layer surface using the four types of coatings made of a base resin and a curing agent shown in Table 3 below (Clear Coating, Pearl Coating, Matte Coating and Matte/Pearl Coating), thereby forming a coating layer.

TABLE 3 Paint composition (pbw) Clear Pearl Matte Matte/Pearl Base resin Polyol 27.5 27.5 27.5 27.5 Effect pigment 6.0 6.0 Delustering particles 9.0 9.0 Solvent 72.5 72.5 72.5 72.5 Curing agent HDI isocyanurate 42 42 42 42 Solvent 58 58 58 58

Details on the ingredients in the table are given below.

-   Polyol: A saturated polyester polyol; weight-average molecular     weight (Mw), 28,000; acid value, 4; hydroxyl value, 170 -   Effect pigment: Natural mica (Iriodin 7205, from Sano Paint Co.,     Ltd.) -   Delustering particles: Silica (Finesil X-35, from Maruo Calcium Co.,     Ltd.; average primary particle size, 2.4 μm; BET specific surface     area, 262 m²/g) -   HDI isocyanurate: Available as Duranate™ TPA-100 from Asahi Kasei     Corporation; NCO content, 23.1 wt %; nonvolatiles content, 100 wt % -   Solvent: Ethyl acetate was used as the base resin solvent; ethyl     acetate and butyl acetate were used as the curing agent solvents

Details on the polyurethane resins used in the resin materials for the cover layer are shown below.

-   TPU (1): A thermoplastic polyurethane available from DIC Covestro     Polymer, Ltd. under the trade name “Pandex” (Shore D hardness, 40),     the number-average molecular weight of the polyol component, 1000;     uncolored product -   TPU (2): A thermoplastic polyurethane available from DIC Covestro     Polymer, Ltd. under the trade name “Pandex” (Shore D hardness, 65),     the number-average molecular weight of the polyol component, 1000;     uncolored product -   TPU (3): A thermoplastic polyurethane available from DIC Covestro     Polymer, Ltd. under the trade name “Pandex” (Shore D hardness, 40),     the number-average molecular weight of the polyol component, 2000;     uncolored product -   TPU (4): A thermoplastic polyurethane available from DIC Covestro     Polymer, Ltd. under the trade name “Pandex” (Shore D hardness, 47),     the number-average molecular weight of the polyol component, 2000;     uncolored product -   TPU (5): A thermoplastic polyurethane available from DIC Covestro     Polymer, Ltd. under the trade name “Pandex” (Shore D hardness, 53),     the number-average molecular weight of the polyol component, 2000;     uncolored product -   TPU (6): A thermoplastic polyurethane available from DIC Covestro     Polymer, Ltd. under the trade name “Pandex” (Shore D hardness, 40),     the number-average molecular weight of the polyol component, 2000;     white colored product -   TPU (7): A thermoplastic polyurethane available from DIC Covestro     Polymer, Ltd. under the trade name “Pandex” (Shore D hardness, 40),     the number-average molecular weight of the polyol component, 2000;     yellow colored product -   TPU (8): A thermoplastic polyurethane available from DIC Covestro     Polymer, Ltd. under the trade name “Pandex” (Shore D hardness, 40),     the number-average molecular weight of the polyol component, 2000;     black colored product     -   In Comparative Example 5, a thermoplastic polyester elastomer,         available as “Hytrel® 4001” from DuPont-Toray Co., Ltd., is used         as the resin material for the cover layer.

The light transmittance of the resulting cover layer in each of the Examples and Comparative Examples, the lightness (Li value) of the resulting intermediate layer-encased sphere, the lightness (Lo value) of the golf ball, and the visibility, distinctive aesthetic appearance (Kawaii level), scuff resistance, and spin performance on approach shots of the golf ball were evaluated according to the following criteria. The results are presented in Tables 4 to 6.

Visible Light Transmittance of Cover Layer

The light transmittance (T %) was measured using the UV-1800 ultraviolet-visible spectrophotometer (Shimadzu Corporation). The cover layer was peeled from a coated ball produced as described above and measurement was carried out using as the measurement sample a thin strip cut from the cover layer. Regarding the light path during measurement, measurement was carried out from the surface side of the cover layer and the slit width was set to 0.1 nm. The average value was determined after data sampling at a 1-nm pitch in the wavelength range of 380 to 780 nm.

L* Value (Lightness)

The lightness (L* value) was determined based on the L*a*b* color space in JIS Z8781 by measurement using a color difference meter (model SC-P, from Suga Test Instruments Co., Ltd.). A larger value indicates a lighter color tone. In the practice of the invention, the coated ball was set in the instrument and measured. Hence what was being measured was the appearance of the ball itself.

Also, with regard to the measurement for the lightness (Li value) of the intermediate layer, in order to measure a color tone of an intermediate layer surface in the state of encasing the core with the intermediate layer, such the intermediate layer-encased sphere was set in the instrument and measured.

Evaluation of Ball Appearance (Kawaii Level)

Ten golfers carried out sensory evaluations of the golf balls in each Example on the following five-point scale.

Score

-   -   1 point: Appearance is ordinary; ball does not feel cute.     -   2 points: Ball feels somewhat cute and okay to use.     -   3 points: Ball feels cute and appearance is good; would like to         try using.     -   4 points: Ball feels very cute and appearance is very good;         would like to use.     -   5 points: Ball feels really cute and appearance is especially         good; would very much like to use.

Ball Visibility

Ten skilled amateur golfers carried out sensory evaluations of the golf balls in each Example on the ease of seeing the trajectory of the ball when hit with a driver (W #1) in fair weather. The evaluations were carried out based on the following criteria.

-   -   Exc: Eight or more of ten golfers felt that ball trajectory when         hit was easy to see.     -   Good: Five to seven of ten golfers felt that ball trajectory         when hit was easy to see.     -   Fair: Three or four of ten golfers felt that ball trajectory         when hit was easy to see.     -   NG: Two or fewer of ten golfers felt that ball trajectory when         hit was easy to see.

Surface Hardness of Ball

The surface hardness of the ball in each Example was measured with a JIS-C durometer by perpendicularly pressing the indenter of the durometer against the ball surface. The surface hardness of a ball (the surface hardness of the cover) is a measured value obtained at a land (non-dimple) area on the ball surface.

Spin Performance on Approach Shots

A sand wedge (SW) was mounted on a golf swing robot and the rate of backspin by the ball immediately after being struck at a head speed (HS) of 11 m/s was measured with an apparatus for measuring the initial conditions.

Scuff Resistance

The golf balls were held isothermally at 23° C. and five balls of each type were hit at a head speed of 33 m/s using as the club a pitching wedge mounted on a swing robot machine. The damage to the ball from the impact was visually rated.

-   -   Exc: No damage or substantially no apparent damage.     -   Good: Damage is apparent but so slight as to be of substantially         no concern.     -   Fair: Some fraying of surface or loss of dimples, but, it can be         used.     -   NG: Dimples completely obliterated in places, and thus it is no         worth using.

TABLE 4 Example 1 2 3 4 5 6 7 8 9 Cover resin Cover resin material TPU TPU TPU TPU TPU TPU TPU TPU TPU compound (1) (1) (1) (1) (1) (1) (1) (1) (1) Number-average 1000 1000 1000 1000 1000 1000 1000 1000 1000 molecular weight of PTMG Shore D hardness 40 40 40 40 40 40 40 40 40 Color of Cover layer clear clear clear clear clear clear clear clear clear Intermediate layer (Color) white pink green yellow red white yellow white yellow Coating clear clear clear clear clear pearl pearl matte matte Visible light transmittance T (%) 12.7 10.3 10.7 11.8 11.6 9.1 8.8 3.8 4.2 Lightness (Lo) of Ball 86.1 57.4 83.6 82.2 43.8 86.2 95.7 86.0 81.4 Lightness (Li) of intermediate layer- 88.7 60.2 88.3 86.0 46.3 88.7 86.0 88.7 86.0 encased spheres Lo/Li 0.97 0.95 0.95 0.96 0.95 0.97 1.11 0.97 0.95 Visibility Exc Exc Exc Exc Exc Exc Exc Exc Exc Kawaii level (KWI score) 4.8 5.0 4.8 4.8 5.0 4.8 4.9 4.9 4.9 Spin rate on 15 yard approach shot 4,804 4,761 4,826 4,835 4,783 4,757 4.816 4,762 4,849 (rpm) Scuff resistance Good Good Good Good Good Good Good Good Good

TABLE 5 Example 10 11 12 13 14 15 16 17 Cover resin Cover resin material TPU TPU TPU TPU TPU TPU TPU TPU compound (1) (1) (2) (2) (3) (3) (4) (5) Number-average 1000 1000 1000 1000 2000 2000 2000 2000 molecular weight of PTMG Shore D hardness 40 40 65 65 40 40 47 53 Color of Cover layer clear clear clear clear clear clear clear clear Intermediate layer (Color) white yellow white yellow white yellow yellow yellow Coating matte matte clear clear clear clear clear clear pearl pearl Visible light transmittance T (%) 7.4 7.3 6.9 6.5 1.9 3.2 1.8 0.6 Lightness (Lo) of Ball 86.7 98.3 83.3 80.7 87.9 85.3 86.2 88.4 Lightness (Li) of intermediate layer- 88.7 86.0 88.7 86.0 88.7 86.0 86.0 86.0 encased spheres Lo/Li 0.98 1.14 0.94 0.94 0.99 0.99 1.00 1.03 Visibility Exc Exc Exc Exc Exc Exc Exc Exc Kawaii level (KWI score) 4.8 4.8 4.8 4.8 4.0 4.5 4.5 4.8 Spin rate on 15 yard approach shot 4,786 4,788 4,172 4,149 5,087 5,092 4,821 4,593 (rpm) Scuff resistance Good Good Good Good Exc Exc Exc Exc

TABLE 6 Comparative Example 1 2 3 4 5 6 Cover resin Cover resin material TPU TPU TPU TPU TPE Commercial compound (6) (6) (7) (8) name (1) Number-average 2000 2000 2000 2000 — — molecular weight of PTMG Shore D hardness 40 40 40 40 — — Color of Cover layer white white yellow black clear white Intermediate layer (Color) white white white white white white Coating clear pearl pearl clear clear clear Visible light transmittance T (%) 0.0 0.0 0.5 0.0 1.7 0.0 Lightness (Lo) of Ball 88.1 88.6 97.9 20.5 83.8 88.9 Lightness (Li) of intermediate layer-encased 88.7 88.7 88.7 88.7 88.7 — spheres Lo/Li 0.99 1.00 1.10 0.23 0.94 — Visibility Exc Exc Exc NG Exc Exc Kawaii level (KWI score) 1.0 1.2 1.3 1.8 4.8 1.0 Spin rate on 15 yard approach shot (rpm) 5,090 5,088 5,072 5,105 5,121 4,870 Scuff resistance Exc Exc Exc Exc NG Exc Commercial name (1): Trade name - Titleist ProV1 (2019 model) (product of Acushnet Co.)

The following is apparent from the evaluation results in Tables 4 to 6.

In cases where the hardness of the urethane resin material used in the cover layer differs, using a relatively soft resin material results in a ball appearance (impression) that has a high transparency and a clear appearance. Moreover, the spin properties are good and, because a polyurethane resin is used, the scuff resistance of the ball is also good. In addition, it is seen that when poly tetramethylene glycol (PTMG) having small number-average molecular weight (Mn) is used as a long-chain polyol in polyurethane resin, a distinctive aesthetic appearance of the golf ball become good and can obtain a good scuff resistance, compared with using PTMG having large number-average molecular weight.

Japanese Patent Application No. 2019-226473 is incorporated herein by reference.

Although some preferred embodiments have been described, many modifications and variations may be made thereto in light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described without departing from the scope of the appended claims. 

1. A golf ball comprising a core, a cover layer, an intermediate layer between the core and the cover layer, wherein the cover layer is formed of a resin material composed primarily of polyurethane or polyurea, a sphere obtained by encasing the core with the intermediate layer (intermediate layer-encased sphere) has a lightness (Li value) in the L*a*b* color space of at least 25 on its surface, and the cover layer has a visible light transmittance of at least 0.6%.
 2. The golf ball of claim 1, wherein the ball has a lightness (Lo value) in the L*a*b* color space of at least
 25. 3. The golf ball of claim 2, wherein the ratio of the lightness (Lo value) and the lightness (Li value) of the intermediate layer-encased sphere, expressed by Lo/Li is from 0.3 to 1.5.
 4. The golf ball of claim 1, wherein the resin material of the cover layer has a Shore D hardness of not more than
 65. 5. The golf ball of claim 1, wherein the cover layer is formed of a resin material composed primarily of polyurethane in which a polyol component includes poly(tetramethylene glycol) having the number-average molecular weight of not more than 3,500. 